In the past decade, there has been a rapidly growing need for communication bandwidth from high-performance computing and datacenters (see for example, G. T. Reed, G. Mashanovich, F. Y. Gardes, and D. J. Thomson, “Silicon optical modulators,” Nat. Photonics 4(8), 518-526 (2010)). Silicon photonics technology has shown great potential to become a low cost and reliable solution for next generation interconnects due to its compatibility with CMOS technology (see for example, Y. A. Vlasov, “Silicon CMOS-integrated nano-photonics for computer and data communications beyond IOOG,” IEEE Commun. Mag. 50(2), 67-72 (2012)). However, for silicon photonics technology to be widely adopted, a key challenge that needs to be addressed is achieving efficient and high-speed modulation in silicon, while consuming a minimal amount of die area. To minimize the optical and electrical power consumption, the silicon modulator is expected to have low insertion loss and driving voltage, while operating at high data rates (see for example, Miller, D. “Device requirements for optical interconnects to silicon chips”. Proc. IEEE 97, 1166-1185 (2009)).
Today, carrier-depletion based modulators are among the most competitive approaches for data communication applications due to a relatively simple fabrication process and high operation speed. In this approach, a PN junction is formed inside a rib silicon waveguide by implantation. Optical modulation is obtained via the carrier dispersion effect (see for example, R. A. Soref, and B. R. Bennett, “Electrooptical effects in silicon,” IEEE J. Quantum Electron. 23(1), 123-129 (1987)) by depleting the free carriers in the PN junction. The optical phase modulation can be converted to intensity modulation by structures such as Mach-Zehnder interferometer and ring resonator. Silicon modulators operating at 25 Gb/s and beyond has been demonstrated by several groups based on this idea (see for example, L. Liao, A. Liu, D. Rubin, J. Basak, Y. Chetrit, H. Nguyen, R. Cohen, N. Izhaky, and M. Paniccia, “40 Gbit/s silicon optical modulator for high-speed applications,” Electron. Lett. 43(22), 1196-1197 (2007); T. Baehr-Jones, R. Ding, Y. Liu, A. Ayazi, T. Pinguet, N. C. Harris, M. Streshinsky, P. Lee, Y. Zhang, A. E. Lim, T. Y. Liow, S. H. Teo, G. Q. Lo, and M. Hochberg, “Ultralow drive voltage silicon traveling-wave modulator,” Opt. Express 20(11), 12014-12020 (2012); M. Ziebell, D. Marris-Morini, G. Rasigade, J.-M. Fédéli, P. Crozat, E. Cassan, D. Bouville, and L. Vivien, “40 Gbit/s low-loss silicon optical modulator based on a pipin diode,” Opt. Express 20(10), 10591-10596 (2012); D. J. Thomson, F. Y. Gardes, Y. Hu, G. Mashanovich, M. Fournier, P. Grosse, J.-M. Fedeli, and G. T. Reed, “High contrast 40 Gbit/s optical modulation in silicon,” Opt. Express 19(12), 11507-11516 (2011); J. Ding, H. Chen, L. Yang, L. Zhang, R. Ji, Y. Tian, W. Zhu, Y. Lu, P. Zhou, R. Min, and M. Yu, “Ultra-low-power carrier-depletion Mach-Zehnder silicon optical modulator,” Opt. Express 20(7), 7081-7087 (2012); Long Chen, Christopher R. Doerr, Po Dong, and Young-kai Chen, “Monolithic silicon chip with 10 modulator channels at 25 Gbps and 100-GHz spacing,” Opt. Express 19, B946-B951 (2011); J. C. Rosenberg, W. M. J. Green, S. Assefa, D. M. Gill, T. Barwicz, M. Yang, S. M. Shank, and Y. A. Vlasov, “A 25 Gbps silicon microring modulator based on an interleaved junction,” Opt. Express 20, 26411-26423 (2012); Xi Xiao, Hao Xu, Xianyao Li, Yingtao Hu, Kang Xiong, Zhiyong Li, Tao Chu, Yude Yu, and Jinzhong Yu, “25 Gbit/s silicon microring modulator based on misalignment-tolerant interleaved PN junctions,” Opt. Express 20, 2507-2515 (2012); and Guoliang Li, Xuezhe Zheng, Jin Yao, Hiren Thacker, Ivan Shubin, Ying Luo, Kannan Raj, John E. Cunningham, and Ashok V. Krishnamoorthy, “25 Gb/s 1V-driving CMOS ring modulator with integrated thermal tuning,” Opt. Express 19, 20435-20443 (2011)). A large fraction of these results are based on phase shifters with a simple PN junction geometry, either lateral or vertical inside a waveguide.
One important fact to note about current approaches is that the traveling-wave devices tend to be long—often several mm or more. This is due to the fairly weak electro-optic effect in silicon. It is possible to increase the phase shift per unit voltage (characterized by the FOM VπL) associated with the silicon pn-junction, but only by increasing the dopant concentration, which subsequently raises the waveguide loss. This fundamental tradeoff has been observed elsewhere (see for example, Hui Yu, Marianna Pantouvaki, Joris Van Campenhout, Dietmar Korn, Katarzyna Komorowska, Pieter Dumon, Yanlu Li, Peter Verheyen, Philippe Absil, Luca Alloatti, David Hillerkuss, Juerg Leuthold, Roel Baets, and Wim Bogaerts, “Performance tradeoff between lateral and interdigitated doping patterns for high speed carrier-depletion based silicon modulators,” Opt. Express 20, 12926-12938 (2012); and Hui Yu; Bogaerts, W.; De Keersgieter, A., “Optimization of Ion Implantation Condition for Depletion-Type Silicon Optical Modulators,” Quantum Electronics, IEEE Journal of, vol. 46, no. 12, pp. 1763, 1768, December 2010), and a loss-efficiency figure of merit (see for example, Xiaoguang Tu, Tsung-Yang Liow, Junfeng Song, Mingbin Yu, and Guo Qiang Lo, “Fabrication of low loss and high speed silicon optical modulator using doping compensation method,” Opt. Express 19, 18029-18035 (2011)) (F value) has been introduced to characterize the loss-VπL trade off of the phase shifter. A phase shifter with lower F value is able to achieve the same Vπ with a lower optical insertion loss, which is highly desirable. Therefore, lower F values are better. So far in literature, the F value for a simple junction geometry that does not require high-resolution inter-digitation is typically 10˜30 (see for example, Watts, M. R.; Zortman, W. A.; Trotter, D. C.; Young, R. W.; Lentine, A. L., “Low-Voltage, Compact, Depletion-Mode, Silicon Mach-Zehnder Modulator,” Selected Topics in Quantum Electronics, IEEE Journal of, vol. 16, no. 1, pp. 159, 164, January-February 2010), the lowest reported F value is 10.5 dB-V (see for example, Xi Xiao, Hao Xu, Xianyao Li, Zhiyong Li, Tao Chu, Yude Yu, and Jinzhong Yu, “High-speed, low-loss silicon Mach-Zehnder modulators with doping optimization,” Opt. Express 21, 4116-4125 (2013)) with 1.5 V·cm VπL.
Simply raising the dopant concentrations will actually lead to a less favorable F metric. However, more complex junction geometries such as interleaved junctions and “zigzag” junctions (see for example, Xi Xiao; Xianyao Li; Hao Xu; Yingtao Hu; Kang Xiong; Zhiyong Li; Tao Chu; Jinzhong Yu; Yude Yu, “44-Gb/s Silicon Microring Modulators Based on Zigzag PN Junctions,” Photonics Technology Letters, IEEE, vol. 24, no. 19, pp. 1712, 1714, Oct. 1, 2012)) exhibit more favorable F values. In these designs, the junction area per unit length is intentionally increased to enhance the carrier-light interaction. By this means, 0.24 V·cm VπL with 16 dB/cm optical loss is theoretically predicted (see for example, Zhi-Yong Li, Dan-Xia Xu, W. Ross McKinnon, Siegfried Janz, Jens H. Schmid, Pavel Cheben, and Jin-Zhong Yu, “Silicon waveguide modulator based on carrier depletion in periodically interleaved PN junctions,” Opt. Express 17, 15947-15958 (2009)) (F=3.84 dB-V). These are promising results, but to achieve this ultra low VπL, a 200 nm inter-digitation period is required; this will present difficulties as currently most silicon photonics implant layers are fabricated with lower-resolution masks. So far, the best VπL experimentally achieved in even a 193 nm lithography process is 0.62 V·cm associated with 35 dB/cm optical loss (F=21.7 dB-V). Other methods like compensated doping and PIPIN junction geometry are also been explored in order to reduce the optical loss, however the VπL reported is still relatively high, with F values typically 19 dB-V or higher.
There is a need for optical modulators that did not require high-resolution lithography for their fabrication.